A 'Cosmic Jerk' That Reversed the Universe

By DENNIS OVERBYE

Published: October 11, 2003

Correction Appended

CLEVELAND, Oct. 10—
Astronomers said on Friday that they had determined the time in cosmic history when a mysterious force, ''dark energy,'' began to wrench the universe apart.

Five billion years ago, said Dr. Adam Riess, an astronomer at the Space Telescope Science Institute in Baltimore, the universe experienced a ''cosmic jerk.'' Before then, Dr. Riess said, the combined gravity of the galaxies and everything else in the cosmos was resisting the expansion, slowing it down. Since the jerk, though, the universe has been speeding up.

The results were based on observations by a multinational team of astronomers who used the Hubble Space Telescope to search exploding stars known as Type 1a supernovas, reaching back in time three-quarters of the way to the Big Bang, in which the universe was born. The results should help quell remaining doubts that the expansion of the universe is really accelerating, a strange-sounding notion that has become a pillar of a new and widely accepted model of the universe as being full of mysterious dark matter and even more mysterious dark energy.

''This gives great confidence that we've been on the right track,'' said Dr. Riess, who announced his results at a meeting here on the Future of Cosmology sponsored by the Center for Education and Research in Cosmology and Astrophysics at Case Western Reserve University and the Kavli Institute.

Dr. Lawrence M. Krauss, an astrophysicist at Case Western, called the turnaround from slowing down to speeding up important confirmation.

''The big surprise,'' Dr. Krauss said, ''would have been if it hadn't happened.''

Dr. Joseph Lykken, a physicist at the Fermi National Accelerator Laboratory, known as Fermilab, in Batavia, Ill., said, ''I could go home now and be happy.''

Knowing how and when the jerk occurred, astronomers said, was an important step in figuring out just what the dark energy is.

''He gave us information about when the universe hit the gas pedal,'' said Dr. Michael S. Turner, a cosmologist at the University of Chicago who is director of mathematics and physics at the National Science Foundation. Different theories of dark energy, Dr. Turner said, predict different times for the transition.

''The supernovae have come through,'' Dr. Wendy L. Freedman, director of the Carnegie Observatories in Pasadena, Calif., said. ''The whole result could have disappeared,'' referring to the dark energy acceleration.

A result was also a vindication for Dr. Riess, who was a pivotal member of one of two competing groups, the one led by Dr. Brian P. Schmidt of the Mount Stromlo and Siding Spring Observatories in Australia. The other team was headed by Dr. Saul Perlmutter of the Lawrence Berkeley Laboratory of the University of California, who discovered the cosmic acceleration five years ago. The groups used supernovas to chart the expansion of the universe at different times in the past.

The goal was to measure how much the universe was being slowed by the collective gravity of the cosmos and determine whether the universe would go on forever or recollapse in a ''Big Crunch'' on one distant day.

The groups found, though, that nearby supernovas looked dimmer than they should, implying that the universe was growing faster than expected, speeding up, under the influence of some form of antigravity -- perhaps embedded in the fabric of spacetime itself.

The results were buttressed by studies of radiation left over from the Big Bang that suggested that two-thirds of the mass-energy of the universe resided in this dark energy.

''But there was always a nagging doubt,'' Dr. Riess told his colleagues. If that was the case, supernovas even farther away than the ones already observed should be even dimmer than the supernovas nearby.

On the other hand, if it was really an antigravity energy in space, then as space expanded, the push from this dark energy would grow along with it. In the early years of the universe, the dark energy would have been too small to counteract the gravity of the matter in the universe, and the expansion would have initially slowed. After the universe grew big enough, though, the dark energy would dominate, and the universe would start to expand.

Dr. Riess described the difference between the matter, most of which is dark, and dark energy as, ''One pulls, the other pushes.''

To test which idea was true, Dr. Riess and his colleagues had to find supernovas farther in the past than previous surveys, seven billion light-years ago. In 2001, Dr. Riess and his collaborators found Hubble observations of a supernova 10 billion years in the past. It proved to be anomalously bright, lending credence to the idea that a dark energy had taken over some time in between.

''But a single object is just not robust enough,'' he said. For the last year, he and his colleagues have used the Hubble in collaboration with a large galaxy survey known as Goods to find distant supernovas.

''We found lots of weapons of mass destruction,'' he said, showing Hubble pictures of some exploding with the brilliance of small galaxies 8 billion to 10 billion light-years away.

More important, they were brighter than expected. When he plotted their velocities against distance, or time in the past, Dr. Riess found that the universe had to have changed direction, from slowing to speeding up, over a period of time five billion years ago, the so-called cosmic jerk, using the technical term for a change in acceleration.

''It's great to see it,'' Dr. Riess said.

In Dr. Lykken's words, and as borne out by discussions at the meeting here, ''theorists don't have a clue'' about the identity of the dark energy that is so important.

A leading candidate is something invented by Einstein called the cosmological constant. But physicists despair of understanding Einstein's constant. Efforts to calculate it wind up missing by a factor of 10 60, which has been called the worst mismatch in the history of science.

Another candidate that has lately won some following among theorists is that Einstein was wrong, that the acceleration is sort of an illusion that could be dispelled by modifying his general theory of relativity, the sourcebook of cosmic gravity.

The stakes for the universe are not small. If the cosmological constant is the culprit, the universe will expand faster and faster as time goes on.

''It will be cold and dark in a few billion years,'' Dr. Frank Wilczek, the Herman Feshbach professor of physics at the Massachusetts Institute of Technology, said. ''That would be very sad.''

Photo: Dr. Adam Riess, who reported yesterday on the speeding and expanding universe, at the Space Telescope Science Institute in Baltimore. (Photo by Marty Katz for The New York Times)

Correction: October 13, 2003, Monday Because of an editing error, an article on Saturday about efforts to determine when ''dark energy'' began to accelerate the expansion of the universe misstated a subject about which an astronomer, Dr. Adam Riess, said there was nagging doubt. It was that dust might be making supernovae look dimmer -- not that two-thirds of the mass-energy of the universe resides in dark energy.